1. Field of the Invention
The present invention relates to apparatus for the measurement of linear displacement. The invention, by way of example, is useful in making linear displacement measurements between two mirrored surfaces to determine the flatness of, or angle between the mirrored surfaces. More particularly, the invention relates to optical apparatus which is useful for high accuracy linear displacement metrology using interferometry.
2. The Prior Art
A standard device used in many industries is a stage which moves in two perpendicular directions, designated X and Y. XY stages are used in equipment for processing semiconductor wafers, such as step and repeat cameras. Step and repeat cameras require information on the position of an XY stage. Typical of the metrology of XY stages is to measure the position of a mirror mounted on the moving stage using displacement measuring interferometers. In such metrology systems, the stage mirrors must be at least as long as the range of travel of the stages.
A critical dimension for these stage mirrors is the flatness of the mirrored surface. Flatness errors in such stage mirrors will contribute an error to the displacement measurement and will cause a perfect servo system to follow the shape of the mirror, not a straight line as desired. It may be desirable for a user of XY stages to map the errors of a stage mirror allowing the user to compensate for the errors and/or procure mirrors with greater flatness error at reduced cost.
In order to measure the flatness of such stage mirrors based on the shape of the wavefront, prior art interferometric measuring equipment uses techniques for sensing the shape of the wavefront reflected from a mirror, such as, for example, disclosed in Malacara, D., Optical Shop Testing, J. Wiley and Sons. However, such prior art measuring equipment for measuring stage mirrors is costly since the size of the interferometer, which affects its cost, and which is generally large for this purpose, must be greater than the longest dimension of the stage mirror for accurate measurements. If, however, the mirrors are tested with interferometers whose aperture is smaller than the mirror's, such as by testing at large angles of incidence (typically&gt;45.degree.), then the sensitivity of the wavefront measuring interferometer is reduced. For example, the accuracy of prior art wavefront measuring interferometers known to applicant is limited to greater than 12 nanometers.
Other prior art systems for measuring the flatness of mirrors are disclosed in W. T. Estler, Optical Engineering, 24(3), 372-279 (May/June 1985) and U.S. Pat. No. 4,334,778. In such prior art systems, the displacement measuring interferometer outputs two parallel measurement beams to the stage mirror. A disadvantage of this prior art approach for flatness measurements is that if one beam of the interferometer were to encounter a depression in the stage mirror surface, the interferometer system would sense the depression as counts, and would be unable to unambiguously determine which beam saw the depression.
The flatness of a stage mirror is determined using the present invention by placing the interferometer on the midline between the test stage mirror and a known standard stage mirror. The stage and standard mirror are separated by a distance dictated by the interferometer design. The present invention is translated on the midline between the stage and standard mirror using some type of linear slide. When coupled with electronics appropriate for displacement measuring interferometers, the output of the system will be proportional to the sum of the flatness errors of the standard and stage mirrors. The flatness of the stage mirror is obtained when the flatness of the standard mirror and tilt between the two mirrored surfaces is subtracted from these measurements.
Since the tilt between the two mirrored surfaces that is subtracted for the flatness measurements may be of interest in some applications, the ability of the present invention to measure this parameter is useful.
The present invention overcomes the disadvantages of the prior art by using glass wedges to refract the beams produced by the interferometer onto a single area approximately the size of the interferometer beam. If only one spot is produced on each of the stage and standard mirrors, the aforementioned ambiguity concerning the two beams is eliminated. The stage and standard mirrors are located where the refracted beam crosses an interferometer axis of symmetry.
Measurements by the present invention of dimensions perpendicular to the input beam are insensitive to translation of the interferometer. This occurs because the optical path of one polarization depends only on the distance between the two interferometer reference mirrors and the other depends only on the distance between the standard and stage mirrors.